[codex] deprecate get-or-create container guidance

[zxch3n]

Summary

Deprecates the legacy get-or-create child-container APIs for lazy map-key child creation and updates public guidance to prefer mergeable containers.

Changes

• Mark get_or_create_container / getOrCreateContainer as deprecated while preserving existing behavior.
• Clarify that insert_container / setContainer create regular op-id child containers, and that ensure_mergeable_* / ensureMergeable* should be used for lazy map-key child creation that must converge across peers.
• Update Rust/WASM docs, skill guidance, READMEs, changelogs, encoding docs, and examples.
• Keep legacy behavior tests, with explicit deprecated allowances where those tests intentionally exercise old APIs.

Validation

• cargo check -p loro-internal
• cargo check -p loro
• cargo check -p loro-wasm
• cargo test -p loro --test mergeable_public_api
• cargo test -p loro --test contracts
• cargo test -p loro --test loro_rust_test test_get_or_create_container_with_null
• pnpm -C crates/loro-wasm exec tsc --noEmit
• pnpm -C crates/loro-wasm exec vitest run tests/mergeable.test.ts
• pnpm check

Notes

The website source is not in this worktree, so external loro.dev tutorial updates are not included here.